Reconciliation of the excess 176Hf conundrum in meteorites: Recent disturbances of the Lu-Hf and Sm-Nd isotope systematics

1,2Rebecca Bast et al. (>10)*
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.043]
1Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstr. 24, D-48149 Münster, Germany
2Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Str. 49b, D-50674 Köln, Germany
*Find the extensive, full author and affiliation list on the publishers website
Copyright Elsevier

The long-lived 176Lu-176Hf and 147Sm-143Nd radioisotope systems are commonly used chronometers, but when applied to meteorites, they can reveal disturbances. Specifically, Lu-Hf isochrons commonly yield dates up to ∼300 Myr older than the solar system and varying initial 176Hf/177Hf values. We investigated this problem by attempting to construct mineral and whole rock isochrons for eucrites and angrites. Meteorites from different parent bodies exhibit similar disturbance features suggesting that a common process is responsible. Minerals scatter away from isochron regressions for both meteorite classes, with low-Hf phases such as plagioclase and olivine typically being most displaced above (or left of) reference isochrons. Relatively Hf-rich pyroxene is less disturbed but still to the point of steepening Lu-Hf errorchrons. Using our Lu-Hf and Sm-Nd data, we tested various Hf and Lu redistribution scenarios and found that decoupling of Lu/Hf from 176Hf/177Hf must postdate the accumulation of significant radiogenic 176Hf. Therefore early irradiation or diffusion cannot explain the excess 176Hf. Instead, disturbed meteorite isochrons are more likely caused by terrestrial weathering, contamination, or common laboratory procedures. The partial dissolution of phosphate minerals may predominantly remove rare earth elements including Lu, leaving relatively immobile and radiogenic Hf behind. Robust Lu-Hf (and improved Sm-Nd) meteorite geochronology will require the development of chemical or physical methods for removing unsupported radiogenic Hf and silicate-hosted terrestrial contaminants without disturbing parent-daughter ratios.

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